Eliminating a warning for unused variable in non-debug build.

sql/log_event.cc

@@ -5906,7 +5906,9 @@ unpack_row(RELAY_LOG_INFO const *rli,
         /*
           We only unpack the field if it was non-null
         */
+#ifndef DBUG_OFF
         const char *const old_ptr= pack_ptr;
+#endif
         pack_ptr= f->unpack(f->ptr, pack_ptr);
         DBUG_PRINT("debug", ("Unpacking field '%s' from %d bytes",
                              f->field_name, pack_ptr - old_ptr));